Electronic products have recently dealt with an enormous amount of data according to the market demand.
Although only singles were transmitted in the past, a large amount of data may be very quickly transmitted by using differential signals at present.
A computer, an example of such a digital electronic product, has dealt with high resolution images or moving images and thus requiring a signal transmission system faster than other ones.
This is a background of commercializing a common mode filter (CMF) for a fast signal transmission, such as a display port.
FIG. 1 is a table of effective frequencies for respective signal transmission circuits. A universal serial bus (USB) 3.0 super speed (SS) uses an ultra high frequency band of 2.5 GHz and thus requiring a CMF capable of high frequency transmission. As a result, it was important as to where an element for limiting a cut-off frequency is disposed in the CMF.
In general, the CMF is configured to, as shown in FIG. 2, allow two coils to turn in the same direction with respect to a center magnetic path (a magnetic core) and extend to be coupled to terminal portions formed at both edges.
As shown in FIG. 3A, since signals flow in two coils in the same turn direction by signals applied from a terminal portion formed at one side in a common mode, impedance components are summed with respect to a magnetic core, and thus a high impedance component is formed.
As shown in FIG. 3B, since signals flow in two coils in different turn direction by signals applied from terminal portions formed at both sides in a differential mode, impedance components are offset with respect to a magnetic core. In other words, coils that turn with respect to a center magnetic core are formed at different phases, and thus the coils function to offset a magnetic flux, and directions having the same phase are formed in terminal portions, and thus an opposite characteristic to that of the common mode exhibits.
In such a conventional structure, it is very important to increase an electromagnetic degree of coupling between a primary coil and a secondary coil so as to enhance an electric characteristic, and an interval between the two coils needs to be reduced or a magnetic path needs to be formed not to generate a leakage magnetic flux so as to increase the electromagnetic degree of coupling between the primary coil and the secondary coil.
However, terminal portions for mounting lean to edges as surface mounting portions, and thus the conventional structure is to be a structure in which a matching relation between coils is not established.
Referring to FIG. 2, when the terminal portions are formed at edges, one of the terminal portions may have 0.5 turn more or less than the other one in a center magnetic core structure, which causes a distance difference between the terminal portions, and thus impedance between terminals of the two coils may not be structurally the same.
As a result, an impedance difference between terminals occurs in the common mode of FIG. 3A, and an unnecessary impedance (parasitic impedance) component is generated in the differential mode of FIG. 3B.
Accordingly, although manufactures compensate for coil turns between coils by using diverse methods, they fail to establish a perfect matching relation.